Currently, various organic materials such as polyimide, acrylic, and polyvinyl phenol are studied, from which gate insulating films of thin film transistors can be formed by application. Such organic materials are practically used in various fields for insulating film materials except for the gate insulating films and can be usually used in a form of a film thickness of 1 μm or more. The reason why such organic materials are used as a film having a film thickness of 1 μm or more is that a film having a larger thickness can reduce leak current as well as increase insulating breakdown voltage. Furthermore, a large film thickness is preferable from the viewpoint of parasitic capacitance reduction in a device.
In contrast, a gate insulating film of an organic transistor is the insulating film for inducing charge in an organic semiconductor layer. Thus, in order to increase charge density in the organic semiconductor layer, the insulating film preferably has a smaller film thickness. Hence, it is difficult to employ the organic materials described above having a film thickness of 1 μm or more as they are for the material for gate insulating films.
Furthermore, the operation of an organic transistor requires the application of a very high electric field of 1 MV/cm or more to a gate insulating film. Most of electronic devices other than the organic transistor do not require such a high electric field, but the gate insulating film of an organic transistor requires extremely high performance as compared with insulating films for other applications.
Thus, the gate insulating film of an organic transistor requires to have a film thickness of 1 μm or less as well as to satisfy a low leak current density (for example, a volume resistivity of 1015 Ωcm or more (when 1 MV/cm)) and a high insulating breakdown voltage (1 MV/cm or more).
Moreover, it is known that surface free energy (water contact angle) and polarity of the gate insulating film affect mobility of the organic transistor because the gate insulating film is in contact with an organic semiconductor layer of the organic transistor (Non-patent Document 1). In order to increase the mobility, it is believed that the gate insulating film preferably has a flat surface and a low surface free energy. However, the gate insulating film having a too low surface free energy interferes with the formation of a film of an organic semiconductor using a coating solution of an organic semiconductor.
It is further believed that water and oxygen in air also reduce characteristics of the organic transistor (Non-patent Document 2). In addition to the case of the adsorption of water or oxygen into the organic semiconductor layer, water or oxygen may be adsorbed into the gate insulating film. Thus, in order to stabilize an on/off ratio, hygroscopic properties and oxygen adsorption properties of the gate insulating film should be taken into account.
Therefore, material design for the gate insulating film of an organic transistor should be performed in consideration of the characteristics of the organic transistor along with the basic performance of the insulating film. Thus, it is difficult to use an existing insulating film material without any modification.
Furthermore, the most serious problem in the practical use of the organic transistor is heat resistance of a plastic substrate. That is comparatively cheap plastic substrates such as PEN (polyethylene naphthalate) and PET (polyethylene terephthalate) have low heat resistance, and thus other members such as gate insulating films that are formed on the plastic substrate have a limited baking temperature due to the low heat resistance of the plastic substrate. On this account, for example, Non-patent Document 3 discloses a polyimide precursor to be cured at low temperature. However, the polyimide precursor can be dissolved only in some limited solvents such as amide solvents. Therefore, the polyimide precursor is not considered as a insulating film forming material that is soluble in a wide variety of solvents.